Rigidity percolation uncovers a structural basis for embryonic tissue phase transitions

The a 0 × b 0 × 2c 0 twofold superstructure of dicaesium mercury tetrachloride, Cs2HgCl4, at T = 120 K has been determined by single-crystal X-ray diffraction using synchrotron radiation. Lattice parameters were found as a = 9.7105 (2), b = 7.4691 (1), c = 26.8992 (4) Å, and β = 90.368 (1)° with the supercell space group P21/c. Refinements on 1828 observed unique reflections converged to R = 0.053 (wR = 0.057) using anisotropic temperature factors for all atoms. This phase is the stable phase of Cs2HgCl4 below 163 K. A quantitative comparison is made of the distortions of the 2c 0 superstructure with the undistorted phase that is stable at room temperature, and with the 3c 0 and 5a 0 superstructures that are stable at temperatures between 163 K and room temperature. The principal difference between the 2c 0 superstructure and all other phases of Cs2HgCl4 is that the Cs cations are displaced away from the centers of their coordination polyhedra in the 2c 0 superstructure. The structural basis for the driving force of the series of phase transitions in this compound is found in the variations of the environments of Cs atoms and in the variations of the distortions of the HgCl4 tetrahedra.

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Electron Microscopy of Graphite Intercalation Compounds

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100055564 ◽

1982 ◽

Vol 40 ◽

pp. 544-545

Author(s):

G. Timp ◽

L. Salamanca-Riba ◽

L.W. Hobbs ◽

G. Dresselhaus ◽

M.S. Dresselhaus

Keyword(s):

Electron Microscopy ◽

Phase Transitions ◽

Domain Wall ◽

Intercalation Compounds ◽

Lattice Plane ◽

Wall Model ◽

Graphite Intercalation Compounds ◽

Fundamental Symmetry ◽

Graphite Intercalation

Electron microscopy can be used to study structures and phase transitions occurring in graphite intercalations compounds. The fundamental symmetry in graphite intercalation compounds is the staging periodicity whereby each intercalate layer is separated by n graphite layers, n denoting the stage index. The currently accepted model for intercalation proposed by Herold and Daumas assumes that the sample contains equal amounts of intercalant between any two graphite layers and staged regions are confined to domains. Specifically, in a stage 2 compound, the Herold-Daumas domain wall model predicts a pleated lattice plane structure.

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Organization of tight junctions in the choroid plexus epithelium

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100082546 ◽

1978 ◽

Vol 36 (2) ◽

pp. 336-337

Author(s):

B. Van Deurs ◽

J. K. Koehler

Keyword(s):

Choroid Plexus ◽

Present Report ◽

Freeze Fracture ◽

Barrier Properties ◽

Cell Junctions ◽

Structural Basis ◽

Apical Surface ◽

Choroid Plexus Epithelium ◽

Solid Nitrogen ◽

Special Composition

The choroid plexus epithelium constitutes a blood-cerebrospinal fluid (CSF) barrier, and is involved in regulation of the special composition of the CSF. The epithelium is provided with an ouabain-sensitive Na/K-pump located at the apical surface, actively pumping ions into the CSF. The choroid plexus epithelium has been described as “leaky” with a low transepithelial resistance, and a passive transepithelial flux following a paracellular route (intercellular spaces and cell junctions) also takes place. The present report describes the structural basis for these “barrier” properties of the choroid plexus epithelium as revealed by freeze fracture.Choroid plexus from the lateral, third and fourth ventricles of rats were used. The tissue was fixed in glutaraldehyde and stored in 30% glycerol. Freezing was performed either in liquid nitrogen-cooled Freon 22, or directly in a mixture of liquid and solid nitrogen prepared in a special vacuum chamber. The latter method was always used, and considered necessary, when preparations of complementary (double) replicas were made.

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High resolution spot scan imaging of frozen, hydrated actin bundles with 400kV electrons

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100122964 ◽

1992 ◽

Vol 50 (1) ◽

pp. 512-513

Author(s):

J. Jakana ◽

M.F. Schmid ◽

P. Matsudaira ◽

W. Chiu

Keyword(s):

High Resolution ◽

Binding Proteins ◽

Actin Binding ◽

Eukaryotic Cells ◽

Dimensional Structure ◽

Structural Basis ◽

Actin Bundle ◽

Actin Bundles ◽

3 Dimensional ◽

Macromolecular Assembly

Actin is a protein found in all eukaryotic cells. In its polymerized form, the cells use it for motility, cytokinesis and for cytoskeletal support. An example of this latter class is the actin bundle in the acrosomal process from the Limulus sperm. The different functions actin performs seem to arise from its interaction with the actin binding proteins. A 3-dimensional structure of this macromolecular assembly is essential to provide a structural basis for understanding this interaction in relationship to its development and functions.

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Electron Microscopy and image reconstruction reveal the structural basis for spectrin's elastic properties

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100122952 ◽

1992 ◽

Vol 50 (1) ◽

pp. 510-511

Author(s):

Amy M. McGough ◽

Robert Josephs

Keyword(s):

Electron Microscopy ◽

Elastic Properties ◽

Conformational Changes ◽

Quaternary Structure ◽

Three Dimensional ◽

Membrane Skeleton ◽

Projection Algorithm ◽

Structural Basis ◽

Iterative Approximation ◽

Membrane Skeletons

The remarkable deformability of the erythrocyte derives in large part from the elastic properties of spectrin, the major component of the membrane skeleton. It is generally accepted that spectrin's elasticity arises from marked conformational changes which include variations in its overall length (1). In this work the structure of spectrin in partially expanded membrane skeletons was studied by electron microscopy to determine the molecular basis for spectrin's elastic properties. Spectrin molecules were analysed with respect to three features: length, conformation, and quaternary structure. The results of these studies lead to a model of how spectrin mediates the elastic deformation of the erythrocyte.Membrane skeletons were isolated from erythrocyte membrane ghosts, negatively stained, and examined by transmission electron microscopy (2). Particle lengths and end-to-end distances were measured from enlarged prints using the computer program MACMEASURE. Spectrin conformation (straightness) was assessed by calculating the particles’ correlation length by iterative approximation (3). Digitised spectrin images were correlation averaged or Fourier filtered to improve their signal-to-noise ratios. Three-dimensional reconstructions were performed using a suite of programs which were based on the filtered back-projection algorithm and executed on a cluster of Microvax 3200 workstations (4).

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Electron diffraction detection of ordliking in annealed PbTe thin film

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100178227 ◽

1990 ◽

Vol 48 (4) ◽

pp. 1018-1019

Author(s):

Karimat El-Sayed

Keyword(s):

Thin Film ◽

Electron Diffraction ◽

Lead Telluride ◽

Structural Basis ◽

Face Centered Cubic ◽

X Ray ◽

Polycrystalline Thin Films ◽

Stage Process ◽

Diffraction Patterns ◽

Face Centered

Lead telluride is an important semiconductor of many applications. Many Investigators showed that there are anamolous descripancies in most of the electrophysical properties of PbTe polycrystalline thin films on annealing. X-Ray and electron diffraction studies are being undertaken in the present work in order to explain the cause of this anamolous behaviour.Figures 1-3 show the electron diffraction of the unheated, heated in air at 100°C and heated in air at 250°C respectively of a 300°A polycrystalline PbTe thin film. It can be seen that Fig. 1 is a typical [100] projection of a face centered cubic with unmixed (hkl) indices. Fig. 2 shows the appearance of faint superlattice reflections having mixed (hkl) indices. Fig. 3 shows the disappearance of thf superlattice reflections and the appearance of polycrystalline PbO phase superimposed on the [l00] PbTe diffraction patterns. The mechanism of this three stage process can be explained on structural basis as follows :

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Multiple phase transitions investigated by high-speed TEM

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100175880 ◽

1990 ◽

Vol 48 (4) ◽

pp. 550-551

Author(s):

Oleg Bostanjoglo ◽

Peter Thomsen-Schmidt

Keyword(s):

Phase Transitions ◽

High Speed ◽

Short Exposure ◽

Semiconductor Film ◽

Time Resolved ◽

Short Exposure Time ◽

Multiple Phase ◽

Model Substance ◽

History Of ◽

Electron Image

Thin GexTe1-x (x = 0.15-0.8) were studied as a model substance of a composite semiconductor film, in addition being of interest for optical storage material. Two complementary modes of time-resolved TEM were used to trace the phase transitions, induced by an attached Q-switched (50 ns FWHM) and frequency doubled (532 nm) Nd:YAG laser. The laser radiation was focused onto the specimen within the TEM to a 20 μm spot (FWHM). Discrete intermediate states were visualized by short-exposure time doubleframe imaging /1,2/. The full history of a transformation was gained by tracking the electron image intensity with photomultiplier and storage oscilloscopes (space/time resolution 100 nm/3 ns) /3/. In order to avoid radiation damage by the probing electron beam to detector and specimen, the beam is pulsed in this continuous mode of time-resolved TEM,too.Short events ( <2 μs) are followed by illuminating with an extended single electron pulse (fig. 1c)

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